1. Technical Field
The present invention relates to a driving circuit for a capacitive load and a fluid ejecting device and, in particular, to an ink jet recording head that drives a piezoelectric element in response to a trapezoidal-wave drive signal and an ink jet recording device including the ink jet recording head.
2. Related Art
An ink jet recording device is known as a fluid ejecting device that ejects a fluid in response to a drive signal to a target for printing or other purposes. The ink jet recording device includes an ink jet recording head that ejects ink drops through a nozzle aperture in response to a pressure caused by a displacement of a piezoelectric element. Such a fluid ejecting device is supplied with a sufficient level of current to cause a large number of piezoelectric elements to operate smoothly. The fluid ejecting device thus uses a drive signal that is current-amplified by a current amplifier.
When the current amplifier current-amplifies a drive signal, a power consumption of a charging transistor is a product obtained from multiplying a current by a difference between a power source voltage and a voltage of the drive signal. On the other hand, a power consumption of a discharging transistor is obtained from multiplying a current by a difference between the voltage of the drive signal and the ground voltage. The power consumption of each transistor increases. Accordingly, the need for a technique of reducing the power consumption is mounting. Japanese Unexamined Patent Application Publication No. JP-A-2006-272907 discloses a technique of reducing power consumption caused by driving current.
A driving circuit disclosed in Japanese Unexamined Patent Application Publication JP-A-2006-272907 generates an auxiliary drive signal that is offset by a predetermined amount in a manner such that the auxiliary drive signal becomes similar to a main trapezoidal drive signal that drives a piezoelectric element. The auxiliary drive signal is used as a power source voltage in order to reduce a difference between the main drive signal and the auxiliary drive signal. The power consumption is thus reduced.
The driving circuit thus includes a main drive signal generator that generates the main drive signal for a transistor pair in response to an analog signal, and an auxiliary drive signal generator that generates the auxiliary drive signal in response to a pulse signal with another transistor and a smoothing circuit. A comparator in a pulse-width modulation (PWM) circuit is used to obtain the pulse signal by comparing a signal representing the main drive signal with a triangular wave.
The PWM circuit of the driving circuit disclosed in Japanese Unexamined Patent Application Publication JP-A-2006-272907 adds an offset value to a signal to be compared with the triangular signal so that the signal to be compared has an offset to the main drive signal. A delay or the like in the smoothing circuit reduces the difference between the main drive signal and the auxiliary drive signal, and the operation of the PWM may be unstable. The frequency of the main drive signal has been higher and higher recently, and the effect of delay cannot be neglected accordingly. If the offset value between the main drive signal and the auxiliary drive signal is set to be larger from the start, the reduction of the power consumption by reducing a thermal loss in the transistor pair becomes difficult to achieve.